1. Field of the Invention
The present invention relates to ceramic electronic components. In particular, the present invention relates to the structure of a terminal portion and an improvement in material of a ceramic electronic component, such as a monolithic ceramic capacitor, which includes a ceramic electronic component body (hereinafter referred to as a component body) and is surface-mounted.
2. Description of the Related Art
FIG. 1 is a cross-sectional view of a surface-mounted ceramic electronic component 1 which is one of interest in the present invention. The ceramic electronic component 1 includes a rectangular parallelepiped component body 2 having two end faces 3 and 4 opposing each other and four side faces 5 which connect the end faces 3 and 4.
Terminal electrodes 6 and 7 are formed on the end faces 3 and 4, respectively. The terminal electrodes 6 and 7 are formed by, for example, coating and baking a conductive paste, and extend over edge portions of the side faces 5.
The component body 2 is a main constituent of a monolithic ceramic capacitor and includes a plurality of internal electrodes 8 and 9 which are alternately arranged in the interior thereof. The internal electrodes 8 are electrically connected to the terminal electrode 6, whereas the internal electrodes 9 are electrically connected to the terminal electrode 7.
A mounting board 10 for mounting the ceramic electronic component 1 has conductive lands 11 and 12 corresponding to the terminal electrodes 6 and 7, respectively. In surface mounting of the ceramic electronic component 1 onto the mounting board 10, the terminal electrodes 6 and 7 are aligned onto the conductive lands 11 and 12, respectively, and the terminal electrodes 6 and 7 are soldered to the conductive lands 11 and 12, respectively, for example, by a solder reflow process. In the drawing, reference numerals 13 and 14 represent solder provided by the solder reflow process. The solder 13 and 14 is provided on the end faces 3 and 4, respectively, including the extensions which extend over the edge portions of the side faces 5.
In the above surface mounting of the ceramic electronic component 1, distortion of the mounting board 10 and thermal shock cycles involving repeated rising and falling of temperature cause relatively large stresses in the terminal electrodes 6 and 7 and the component body 2. As a result, cracks 15 form in the component body 2, as shown in FIG. 1.
Further, as shown in FIG. 2, there is a case in which solders 13 and 14 are provided not only on the end surfaces 3 and 4 of the terminal electrodes 6 and 17, but also on a side face 5a. In such a case a crack 15 may be provided on the side face 5a side.
Since the stresses causing such cracks 15 particularly affect the component body 2 at the extensions of the terminal electrodes 6 and 7 over the side faces 5, 5a, the cracks 15 readily form in the vicinity of edges 16 and 17 of the terminal electrodes 6 and 7, respectively. Moreover, the solder 13 and 14 provided on the extensions of the terminal electrodes 6 and 7 over the side faces 5, 5a enhances the stress.
When the mounting board 10 is a metal-core mounting board, such as an aluminum board composed of an aluminum base covered with an insulating coating, a large difference in thermal expansion coefficients between the mounting board 10 and the component body 2 causes a large stress during the thermal shock cycles, and thus the cracks 15 readily form.
When the component body 2 is a high-capacitance monolithic ceramic capacitor composed of a Pb-based ceramic dielectric material, the component body 2 has a relatively low bending strength. Thus, the cracks 15 more readily form.
The cracks 15 cause a decrease in humidity resistance and a decrease in thermal shock resistance of the ceramic electronic component 1. Moreover, the cracks 15 cause decreased electrical characteristics such as insulation resistance. As a result, the ceramic electronic component 1 is less reliable.
In order solve the above problems, a conductive resin composed of a resin and metal powder is used for the formation of the terminal electrodes 6 and 7 so that the deformable conductive resin can relax the stress. However, the adhesive strength of the conductive resin of the terminal electrodes 6 and 7 to the component body 2 decreases after the ceramic electronic component 1 is placed in a high-temperature atmosphere, e.g., of approximately 150xc2x0 C., for a long period or in a high-temperature, high-humidity environment, of e.g., at 85xc2x0 C. and an 86% humidity. As a result, the terminal electrodes 6 and 7 are easily detached from the component body 2.
FIG. 3 is a cross-sectional view of another ceramic electronic component 1a which is one of interest in the present invention. In FIG. 3, elements corresponding to the elements shown in FIG. 1 are referred to with the same reference numerals and a repeated description is omitted. The ceramic electronic component 1a is provided in order to solve the above problems regarding the cracks 15. In the ceramic electronic component 1a, a resin coat 18 is applied over the extensions of the terminal electrodes 6 and 7 and the side faces 5. Thus, the solder 13 and 14 is provided only on the end faces 3 and 4 of the terminal electrodes 6 and 7, respectively.
When the ceramic electronic component 1a is mounted onto the mounting board 10, the solder 13 and 14 is not provided on the extensions of the terminal electrodes 6 and 7 on the side faces 5. Thus, the resin coat 18 contributes to a decreased stress and prevents the formation of the cracks 15.
The surface mounting of the ceramic electronic component 1a shown in FIG. 3, however, inhibits direct contact of the terminal electrodes 6 and 7 to the conductive lands 11 and 12, respectively, and causes a decreased contact area of the terminal electrodes 6 and 7 with the solder 13 and 14, respectively. Thus, the bonding strength, particularly the shear strength, of the ceramic electronic component 1a to the mounting board 10 is not so high. As a result, the ceramic electronic component 1a may become detached from the mounting board 10.
It is an object of the present invention to provide a ceramic electronic component which does not cause the formation of cracks and decreased shear strength.
The present invention is directed to a ceramic electronic component comprising at least one component body having two end faces opposing each other and side faces connecting the two end faces, and terminal electrodes formed on the component body, each extending from each end face to edge portions of each side face of the component body. The terminal electrode is characterized as follows in order to solve the above problems.
Each of the terminal electrodes comprises a metal layer formed on at least each end face of the component body, and a conductive resin layer for covering at least portions of the side faces of the component body, the conductive resin layer extending from the metal layer including the edge of the metal layer to the portions of the side faces, and comprising a conductive resin containing metal powder and resin. The thickness of the conductive resin layer above the side faces is at least about 10 xcexcm. A metal plating film covers the outer surface of the terminal electrode.
The metal layer ensures sufficiently high bonding strength to the component body and the plating film facilitates soldering to the terminal electrodes of the ceramic electronic component.
The conductive resin layer relaxes the stress due to distortion of a mounting board and thermal shocks so that cracks do not form in the component body. Thus, the ceramic electronic component has highly reliable electrical characteristics.
Preferably, the thickness of the conductive resin layer above the side faces is in a range of about 20 to 70 xcexcm.
When the thickness of the conductive resin layer above the side faces is at least about 20 xcexcm, the stress is more effectively relaxed. On the other hand, a thickness of not more than about 70 xcexcm does not cause a significant decrease in the shear strength and a significant increase in the ESR due to suppressed deterioration of the conductive resin layer.
Preferably, the conductive resin layer extends above each end face of the component body, and the thickness of the conductive resin layer above each end face is not more than about 5 xcexcm.
In such a configuration, the stress is more effectively relaxed. When the thickness of the conductive resin layer above each end face is not more than about 5 xcexcm, the shear strength does not significantly decrease and the ESR does not significantly increase.
In the present invention, the metal layer may be formed by coating and baking a conductive paste containing, for example, Ag, Agxe2x80x94Pd, Ni or Cu.
This metal layer enhances the bonding strength between the metal layer and the component body, and the bonding strength is resistant to large stress due to thermal shock even when the metal layer is thick.
Preferably, the metal plating film comprises a underlying metal film preventing diffusion of solder into the conductive resin layer and a surface metal film having high solderability.
Such a double layer plating film configuration prevents deterioration of the conductive resin layer due to diffusion of the solder and ensures solderability to the terminal electrode.
In the present invention, the component body may be a plurality of ceramic bodies and the ceramic bodies are stacked so that the terminal electrodes are aligned in the same directions.
In the present invention, it is preferable that the component body is a constituent of a monolithic ceramic capacitor.